1. Field of the Invention
The invention relates to a steam power station with a boiler, a steam turbine, a condenser and a bypass line which bypasses the steam turbine by leading directly from the boiler to the condenser. It relates, in particular, to a steam introduction device between the bypass line and the condenser and to the first of two steam passage diaphragms in this steam introduction device.
2. Discussion of Background
During the startup and rundown of a steam power plant and during steam turbine load shedding due to a shutdown of the plant, the steam is not led from the boiler to the steam turbine, since said steam contains too much water and would consequently damage the turbine blading. Instead, the steam is led directly from the boiler into the condenser through a bypass line and a steam introduction device. The steam introduction device serves for expanding the steam and cooling it before it enters the condenser for condensation. The steam flowing in via the bypass line has, on the one hand, a high flow velocity and, on the other hand, a temperature of up to 600.degree. C. By contrast, the temperature prevailing in the condenser is around 40.degree. C. It is therefore expedient to lower the temperature of the steam and its velocity sharply. This also means that the components of the steam introduction device are exposed to a high temperature gradient.
According to publication number CH-T 080 273 of the Brown Boveri Companie, a bypass regulating valve is followed by a two-stage steam introduction device which is arranged in the condenser. The first stage of the steam introduction device consists of a steam passage diaphragm, specifically a perforated diaphragm which is frustoconical and by means of which the hot steam stream is sprayed and fanned out. Downstream of the perforated diaphragm, the latter enters an expansion or cooling chamber. Here, it is cooled by means of cool condensate which is sprayed into the fanned-out steam stream by a plurality of nozzles. In the second stage of the steam introduction device, the steam flows through a second perforated diaphragm, by means of which the steam is distributed in the condenser neck and over the cooling tubes of the condenser.
A perforated diaphragm of the first stage of the steam introduction device is manufactured from a plurality of plane components, specifically a part for the envelope of the cone frustum, a closure part for the vertex of the cone and a transitional part for connection to the end of the bypass line. The orifices in the perforated diaphragm are drilled into the still plane part of the cone envelope, said part subsequently being hot-formed into a cone and welded together. The closure part for the vertex of the cone is then welded to the cone frustum and the transitional part is welded to the end of the bypass line.
In order to ensure that the cone, which has a multiplicity of drilled orifices, has sufficient mechanical stability, relatively large wall thicknesses are necessary. The larger the wall thickness, the higher the thermal stresses. As mentioned, this perforated diaphragm is exposed to a very high temperature gradient. During use, therefore, the considerable temperature gradient from one side of the perforated diaphragm to the other leads, in the case of large wall thicknesses, to correspondingly high thermal stresses, with the result that cracks may form in the material. As early as during the hot-forming process, too, small cracks may form, and these may subsequently increase in size during operation and ultimately lead to a material fracture. Such susceptibility to cracks or fractures is detrimental to the operating reliability of the power plant, since damage to the perforated diaphragm can be rectified only by a repair, with the entire plant being shut down. Furthermore, the cost-intensive production of the perforated diaphragm is a disadvantage. On the one hand, the manufacture of the plurality of individual parts and the welding work for assembling these necessitate a high outlay in terms of fabrication and cost. On the other hand, while the perforated diaphragm is being formed into the cone, the geometry of the drilled orifices is distorted, so that, where appropriate, the orifices have to be remachined.